Electric heating appliance



Aug. 11, 1931. J. A. BOYER ELECTRIC HEATING APPLIANCE Filed Aug. 19,1929 m t M \H W m m. w

employing non -metal Patented Aug. 11, 1931 STATE ammonia-.1) i

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NEW YORK, A CORPORATION OF NEW YORK msmm HEATING APPLIANCE Applicationfled August 19, 1929. Serial No. 886,998.

This invention relates toelectric heating appliances, and moreespecially to contact terminals for electric eating appliances employinrigid resistors as the heating elements. he invention relates especiallyto a contact terminal of an aluminum alloy vparticularly adapted forthis purpose.

In the drawings,-

Figure 1 is a section taken partly in elevation of an electric heatingdevice embodyin my invention;

igpres 2 and 3 are detail elevations showing t e construction of one ofthe contact terminals;

Fi re 4 is a sectional view taken on the line V-IV of Figure 1; and

Figures 5 and 6 are fragmentary elevations showlng forms of contactterminals.

Heating appliances, such as domestic electric room heaters, hot plates,stoves, etc., are often made with rigid non-metallic resistors as theheating elements. These non-metallic resistors are usually made of orstalline refractory conducting material ormed into rods or bars. As anexample of such type of resistors may be mentioned the resistors soldunder the trade name of Globar which are bars consisting principally ofrecrystallized silicon carbide. I

These non-metallic resistor elements are highly refractory and may beoperated at a high temperature and have a long life. The surfaces ofsuch resistors are somewhat rough and present hig spots or projectingcrystals as contrasted with a metallic surface which may be finished toa smooth condition.

one of the great practical difiiculties in ie rigid resistors of thistype has been the provision of suitable contacting meansbetweentheresistor and the metallic terminalmembers; A metallicjcontact'heldagainst the surface'of the resistor bears only on the high p'ointsof-thesomewhat rough resistor surface, and the'current passing from the;

resistor is' therefore conoentratedatrelativesmall contact areas orpoints. f The result y has been a tendency andloc'al overheating. 1

Many attempts havebeen made solvethe ble,

= terminals haveproved etal to the body of the difliculty of providingsatisfactory contacts. In general these have comprised principallymechanical designs for the metal contact member with the incorporationof a piece of heat-resisting metal at the junction of the resistor andcontact terminal, or the ends of the rods have been coated with a filmof' metal by spraying or plating.

Such mechanical devices have included metal caps for the rods,cumbersome split rings and side contacts, wrapping the ends of theresistor elements with heat-resistant wire, and similar mechanical,devices. In such previous attempts principal reliance has been upon ametal which is particularly heat resistant, such as the nickel-chromiumalloys, iron, chromium alloys, nickel, and nickel al-' loys. Thesealloys are hard as well as heat resistant, and in fact, hardness hasbeen considered a desirable property in most of these metals.

Thin metal films ap lied to the ends'of the rod are not durable ecausetheir intimate union with the resistor causes them to assume the sametemperature as the resistor and because they are too thin to form acushion and thus eliminate arcing and local overheating.

Contacts of soft, relatively low melting oint metal, such as aluminum,have also can suggested, and are described and claimed in the copendingapplication of Harold N. Shaw, filed of even date therewith. Metallicaluminum is soft and readily fusithe high points of the resistor surfacetend to embed themselves in the aluminum andgive a greater area ofcontact than I with a hard metal.

so that when used asthe contact pieceor projecting crystal pointsThealuminum melts at approximately 660 centigrade and becomes soft 1 at4 even lower While the .aluminum contact satisfactory where theterminals are-exposed -and can readily'radiate temperatures.

oints o-f the r surface may embed emselves in the metal, and

' or conduct'away' the heat; they are not eninvention rei;asses h Classof; .tei-minals," namely-,' those-fin which the high" .especially toterminals having greater heat ances, the contact terminals are soexposed that the metallic aluminum may be used as a contact material.However, in other appliances the terminals are enclosed or partiallyenclosed or shielded so much hotter than open or exposed terminals. Insuch cases contacting members of the same general nature as aluminum butof a higher softening point are required.

I have found that certain of the aluminum alloys have a higher meltingpoint as well as a softening range prior to complete fusion, and thatthese alloys provide durable contacting members for rigid non-metallicresistors, particularly for intermittent service and in devices whichprevent rapid dissipation of heat from the terminal members. Mostdomestic appliances are subjected to intermittent service inwhich thecurrent that they become is turned on to the resistors for a short timeand then the current shut off, so that the devicesare subjected tocontinued cycles of heating and cooling. This makes the termi- 'nalconditions severe, tending'to cause arcing andlocal overheating.

As examples of such alloys are the aluminum base alloys containing ametal of the iron group, more particularly. the iron family comprisingthe metals iron, nickel and cobalt, preferably up to about 40% of nickelor up to about 30% of iron. Greater amounts of nickel and iron may bealloyed with the aluminum in certain cases where plasticity of the alloyunder heat is not re quired, but in cases where the successful operation of the contact depends upon the high points of the resistorsurface becoming em bedded in the contacting member to prevent excessiveheating, it is desirable that the aboye amounts of nickel and ironshould not be greatly exceeded. This is because of the fact that withthese figures as the upper limits for the nickel or iron, aluminumexists in the resulting alloy in a free-as well as in a. combined state.It is a well recognized fact that when aluminum is alloyedwith a secondmetal, thealuminum may crystallize as substant ally pure metalmicroscopically'intermingled with the other constituents of the alloy,or it may be'combined as an interme'tallic compound or separate as aphase having none of the characteristics of the pure metal. Theintermetallic compounds usually have both physical and chemicalproperties which are entirely different from those of the componentmetals, so that as the percentage composition of the compornd isapproached or exceeded, the properties of the aluminum disappear. In thealloys havin a composition within the limits specified-a ove, a portionof the alupressure. Thus, aluminum-nickel the alloy, the alloys ofaluminum with the metals of the iron family do not possess a lowincipient melting point, and the property of softening at acomparatively low temperature is lost.

Alloys consisting of nickel and aluminum and containing up to about 42%of nickel have an incipient melting or softening temperature ofapproximately 620 centigrade. An alloy containing 40% nickel has a verysmall percentage of molten metal at this tem perature and is notcompletely melted until it reaches approximately l100 centigrade. By avariation of composition in either direction the amount of the lowmelting constituent, aluminum, may be varied, thus changing theeffectual melting point, or the temperature at which the alloy becomestoo soft to retain its shape under the contact alloys containing up toabout 42% nickel retain to a degree the desirable characteristic,plasticity, of pure aluminum while hot, even though they may be brittleat ordinary room temperature. Moreover, these alloys have a solidification range of several hundred degrees centigrade.

A similar relationship obtains with alloys of aluminum and iron. Freealuminum vanish'es as an alloy constituent at approximately 40% iron,but as this composition is approached the alloys become so brittle thatthe contacts are apt to crack in service. I

have found that an aluminum-iron alloy containing 30% iron has anincipient melting point of approximately 048 Centigrade, and is notcompletely molten until approximately l100 centigrade, thus giving aplastic range of approximately 450 centigrade. \Vhile thesealuminum-nickel and, aluminum-iron alloys are somewhat harder than purealuminum, they are nevertheless softer at the operating temperature thanthe iron-chromium and iron-nickel alloys commonly employed as contactterminals and allow the projecting crystals points of the resistors topress into the metal. Moreover, the lower end of their plastic range isreached at a relatively low temperature and one considerably below theirtemperature of complete melting. so that if the current becomesconcentrated at the point of a projecting crystal so that overheatingresults, the metal at this point hecomes plastic and allows the point tobecome ing the local overheating. This capacity of l 7 much superior tothose of the commonly used materials, such as the chrome-iron,chromenickel steels which have heat resistant but hard surfaces, and arefound to last from ten to fifty times as long on a continuousintermittent test which comprises subjecting the resistor and itscontacts to alternate fifteen minute periods of current turned on andoil". The contact terminals of the aluminum alloys as herein describedare an improvement upon and are superior to a plain aluminum ctmtact,particularly when subjected to severe usage or when employed on enclosedterminals or other terminals which become more highly heatedthan theordinary exposed terminals.

Referring to the illustrated embodiment of the invention,reference'numeral 1 indicates a domestic hot plate having non-metallicrigid silicon carbide resistors 2. The resistors are held at each endbycontact terminals 3. As'shown in the drawings, these cont-act term nalsare partially enclosed or shielded by the surrounding parts of the hotplate so that they do not have a good opportunity to radiate heat.

Each contact terminal comprises an alloy steel spring 4 having upperends-bent over to form inwardly directed flanges 5 to hold a contactpiece 6 of an aluminum-nickel or aluminum-iron alloy. This piece 6-ispreferably hollowed out somewhat as indicated at 7 to provide a seat forthe end of the resistor rod 2. The end of this resistor rod is,

'of course, somewhat rough 'because of its crystalline nature. however,permits the roughend of the rod to become embedded in it and furnish agood contact even under the severe conditions of intermittent use towhich such devices are subjected.

In Figure 5 there is illustrated another form of contact; namely, acone-shaped'piece 10 of aluminum alloy yieldingly pressed by spring 11against the end of a resistor'rod 12.

In Figure 6 is illustrated still another ,modiiication of the inventionin which a cup shaped piece 20 of aluminum alloy is pressed by spring 21againstthe'endof a resistor rod While I have specificallyillustrated anddescribed the preferred embodiments of my invention, it is to beunderstood that the pres-v ent invention is not so limited but may beotherwise embodied within the scope of the following claims.

The aluminum alloy,

a 191 3 f v I claim:' I 1. In an electric heating device, thecombination of a rigid resistor, and terminal members having contactsurfacesof an alloy of aluminum and a metal of the iron family and inwhich some of the aluminum is present in a free and uncombined state.

2. In an electric heating device,'the combination of a rigid resistor,and terminal members having contact surfaces of an alu- 5 minum-nickelalloy, the alloy containing some aluminum in a free and uncombinedstate.

3. In an electric heating device, the combination of a rigid resistor,and terminal members having contact surfaces of an aluminum-nickel alloycontaining up to about 12% nickel.

4. In ,an electric heating device, the combination of a rigidresistor,and terminal members having contact surfaces of an alu- 35minum-nickel alloy containing approximately 40% nickel.

5. In an electric heating device, the combination of a rigid resistor,and terminal members having contact surfaces of an aluminum base alloycontaining aluminum in a free state having a temperature interval between its temperature of incipient melting and its temperature ofcomplete melting o more than 400 C. v

6. In an'electric heating device, the combination of a rigid resistor,and terminal members having contact surfaces of an aluminum base alloycontaining some aluminum in the free state and having an incipientmelting point substantially the same as the melting point of purealuminum and a temperature' of complete melting at least 300 C. abovethe melting point of pure aluminum.

7. In an electric heating device, the com bination of a rigid resistorand terminal members having contact surfaces of an alloy of. aluminumand an iron family metal containing some pf the aluminum in a free statewhich softens at a temperature several hundred degrees below its fusionpoint so that projections on the risistor where local overheating occursmay become embedded in the metal. b

8. In an electric heating device, .the combination of a rigid resistor,terminal members having contact surfaces of an alloy ofv aluminum and ametal of the iron family and in which some of the aluminum is present ina free and uncombined state, and means for 'yieldingly holding theterminal members against the resistor.

9. In an electric heating device, the combination of a rigidnon-metallic resistor and terminal members having contact surfaces of analuminum iron alloy having not more than 40% of iron.

i .In testimony whereof I have hereunto set m hand.

y w A. BOYER- 13o

